Filter for hydrostatic transmission

ABSTRACT

In a hydrostatic transmission or integrated hydrostatic transaxle having a housing forming an oil sump and a hydraulic pump and hydraulic motor mounted in the sump and connected through hydraulic porting, a filter housing or other structure defining an internal volume is mounted adjacent to the check valves for the hydraulic porting. Oil passing into this internal volume is filtered to prevent the introduction of contaminants into the hydraulic porting. A chimney is formed as part of the filter housing and extends upwards therefrom, and is open to the internal volume. The chimney also includes an opening to permit air to escape to the oil sump, to reduce the risk of air being introduced into the hydraulic porting through the check valves.

BACKGROUND OF THE INVENTION

This invention relates to the design of hydrostatic transmissions andintegrated hydrostatic transaxles (IHT) and more particularly to animproved filter design for use with such transmissions and transaxles.The discussion of the invention herein will focus on an IHT as thepreferred embodiment; however, it will be understood that this inventioncould be used with other hydrostatic designs as well.

Such an IHT will generally comprise a hydrostatic pump and motor mountedon a center section, where the hydraulic porting is formed into the bodyof the center section. It is generally understood that in an IHT thehydrostatic transmission and the gear elements are generally running inthe same oil sump. This means such contaminants from the rotating gears,such as metal shavings, will be in the oil, and a filter is needed toprevent such contaminants from entering into the hydrostatic porting anddamaging the running surfaces for the rotating cylinder blocks of thehydrostatic transmission. Check valves may be formed in the centersection and opened at various times to permit oil to flow from the mainoil sump into the hydraulic porting formed in the center section. It isgenerally known to provide a filter that covers these check valves orotherwise filters the oil before it can enter the check valves. One suchknown design is shown in U.S. Pat. No. 5,314,387 where the filter is acap placed on the bottom of the center section, and the filter elementis formed around the periphery thereof. A second known design is shownin U.S. Pat. No. 6,185,936 where the filter element is on the bottomsurface of the cap placed over the bottom of the center section.

It is also well known that excess air in an IHT system can cause noiseand early failure of the hydrostatic elements. When an IHT is beingassembled, the air is purged from the system, which means that all ofthe air in the center section and the hydrostatic porting therein isremoved and replaced by hydraulic oil. There are occasions, however, forservicing of such an IHT unit in the field; during such servicing it isdifficult to completely purge air from the IHT unit and in particularfrom the center section. Because of the geometry of the center sectionand oil filter, air may remain trapped therein after the completion ofpurging. The air trapped in the filter will be adjacent to the checkvalves, and thus there is risk that the air will be drawn into thecenter section through the check valves. Furthermore, air can coalescefrom the oil during operation to form bubbles that can also becometrapped adjacent to the check valves.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved filter designwhich includes a filter housing having an escape valve which permits airto escape from the filter housing but which prevents contaminants fromentering. In the preferred embodiment, the filter housing is mounted onthe bottom of an IHT center section, and a tubular stack or chimney isformed with the filter housing to provide a volume for air to flow toduring purging and operation of the unit. This invention may also beused with other hydrostatic designs where the filter is located belowthe check valves of the hydraulic porting. Such porting need not be in astandard center section, but could also be located in a plate or in thetransmission housing itself.

As a further feature of this invention, an opening and poppet assemblyis provided on the filter housing, and preferably at the top of thechimney, to assist in exhausting air out of the chimney and at the sametime to prevent contaminants from entering the filter housing.

The poppet is denser than the oil and thus will tend to sink. There isan upward force on the poppet equal to the weight of the fluid itdisplaces. When air begins to build up adjacent to the poppet in thefilter chimney, the fluid sees the air and the poppet as a single objectand thus the upward force thereon increases as air displaces oil in thechimney. The poppet will open and air will escape from the opening whenthe upward pressure on the air bubble and poppet is sufficient toovercome the relatively negligible weight of the air bubble and poppetand the surface tension of the oil/air interface.

Further benefits and objects of this invention will be apparent to oneof ordinary skill in the art from a review of the drawings in thefollowing detailed description of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a filter housing and filter inaccordance with this invention.

FIG. 2 is a top plan view of the filter assembly in FIG. 1.

FIG. 3 is a cross-sectional side view of the filter assembly shown inFIG. 2, along the lines 3—3.

FIG. 4 is a side view of an integrated hydrostatic transaxle assemblyincorporating the filter assembly in accordance with the presentinvention.

FIG. 5 is a bottom view of an IHT center section and hydraulic motor andmotor shaft assembly showing the filter housing secured thereto, withthe filter mesh removed for clarity.

DETAILED DESCRIPTION OF THE DRAWINGS

As shown in FIG. 1, the filter assembly 20 is comprised of a filterhousing 22 secured to filter frame 24, which includes filter mesh 35, bymeans of snap connectors 34. Filter assembly 20 is designed for use inconnection with an integrated hydrostatic transaxle such as is shown inU.S. Pat. No. 6,185,936, the terms of which are incorporated herein byreference. Since most of the operation of the integrated hydrostatictransaxle is known and is disclosed in the aforementioned patent, itwill not be discussed in detail here. It will be understood that theinvention can be used in connection with other IHT designs as well.

Relevant elements of the IHT design used with the preferred embodimentof this invention are shown in FIGS. 4 and 5. As is known, input shaft46 is driven by a prime mover (not shown) and is engaged to pump 44.Pump 44 and motor 41 are both mounted on center section 40, whichincludes hydraulic porting (not shown) to hydraulically connect pump 44and motor 41 in a known matter. In the preferred embodiment, pump 44 andmotor 41 are of the axial piston design and have rotating cylinderblocks housing a plurality of pistons, although it is understood thatother designs could be used. Motor shaft 43 is engaged to motor 41 androtates therewith. Motor shaft 43 drives motor gear 45 which is engagedto a gear train comprising reduction gears 51 and 52 which are similarlyengaged to a differential input gear 53. The differential assembly isengaged to and drives a plurality of axles (not shown) for the vehicle.This entire structure is mounted inside a housing comprising a mainhousing 39 and a side housing (not shown) which would be secured theretoto form a sealed common sump. It will be understood that other housingdesigns could be used in accordance with the teachings of thisinvention. For example, a housing having an upper casing and a lowercasing joined along a split line parallel to the axles could also beused. It will also be understood that the gearing and differential couldbe different and need not be present. By way of example, this inventioncould be used with a transmission having only a single output axlemounted in the housing and no differential, or it could be used in ahydrostatic transmission without the reduction gearing in the samehousing.

As shown in FIG. 5, check valves 42 a and 42 b are secured to the bottomof center section 40 in a known matter to allow fluid flow into thehydraulic circuit for pump 44 and motor 41. Filter frame 24 is shownwithout mesh 35 in FIG. 5 for purposes of clarity. Filter housing 22includes two openings 27 a and 27 b formed therein for check valves 42 aand 42 b to be secured to the center section. Thus, in this embodiment,the check valves themselves act to secure filter housing 22 to thebottom of center section 40. Filter housing 22 could be secured tocenter section 40 or otherwise retained in the proper place by otherknown methods. Shield 38 is positioned on tabs 37 formed on filter frame24, and provides protection to filter mesh 35 from oil which maydischarge from high pressure bleeds (not shown) formed in check valves42 a and 42 b.

As shown most clearly in FIGS. 1 and 2, filter housing 22 includes achimney 26 which extends vertically therefrom at one end. Chimney 26defines an internal volume 29. It will also be understood that chimney26 could also be formed with another structure forming an internalvolume adjacent to the check valves 42 a and 42 b of the center section.For example, the filter could be located elsewhere in the sump, and oilcould be passed through tubing or other passages to this separatestructure which would store filtered oil for the check valves 42 a and42 b.

If air is in the system, it is likely to collect along the inner topsurface of filter housing 22 adjacent openings 27 a and 27 b. The top offilter housing 22 includes a slanted surface 25 formed therein andadjacent to chimney 26. This surface provides for a flow route for airto leave the main body of filter housing 22 and enter into chimney 26.It also eliminates the risk that a burr or other imperfection formed onthe filter housing, which is preferably made of plastic, would provide abarrier against which air bubbles could be retained. Notches 55 and 56are formed on the top outer surface of filter housing 22, and correspondto matching projections (not shown) on the bottom of center section 40and serve as guides to assist in assembly.

Chimney 26 includes an upper recess 28 formed therein and open to themain oil sump and an opening 31 to allow air to flow into the main oilsump. A poppet 32 comprised of a cap and snap fingers 33 is placed inthe opening 31. Recess 28 thus forms a lip or raised surface aroundopening 31 and acts as a deflector to prevent turbulence in thehydraulic oil from acting to lift poppet 32 off its resting surface.

In the preferred embodiment, chimney 26 is a generally tubular shapewith a circular cross-section having a radius of approximately 0.325 in.and it extends approximately one (1) inch above the top surface offilter housing 22. Similarly, in the preferred embodiment, the topsurface of poppet 32 has a diameter of 0.50 in.; the top surface has athickness of approximately 0.04 in.; and snap fingers 33 areapproximately 0.44 in. in length measured from the underside of the topsurface. Other shapes and arrangements of these elements are possiblewithin the scope of this invention, and these sizes and shapes aredictated primarily by the surface tension of the hydraulic oil. Forexample, the required size and shape of poppet 32 will depend upon theheight of chimney 26 and the properties of the hydraulic oil; a designhaving a shorter chimney, or no chimney with opening 31 located directlyon filter housing 22, will require a poppet having a different size andconfiguration to ensure that it will properly lift off its restingsurface as described herein.

As can be seen in FIG. 3, poppet fingers 33 are shaped to permit thepoppet to move up and down within opening 31. Oil pressure on the top ofpoppet 32 and/or the weight of poppet 32 will prevent oil and thecontaminants therein from entering through opening 31 into the mainfilter housing 22. When air enters into volume 29, an upward pressure iscreated against poppet 32. As the pressure on the bottom of poppet 32becomes sufficiently high, it will lift poppet 32 off of its base topermit the air to escape through opening 31 into the main oil sump.

It is important that the underside of poppet 32 and the top of upperrecess 28 where poppet 32 rests not be flat and parallel, due to theinability of fluid to penetrate the interface between two very flatcomponents, particularly when you attempt to move the two componentsapart in a direction perpendicular to the interface surfaces. In thepreferred embodiment, these parts are manufactured using injectionmolding and the manufacturing tolerances involved prevent such flatnessfrom being a problem. If a manufacturing technique is used which wouldyield relatively flat surfaces, it is recommended that one of the twosurfaces be chamfered or beveled slightly (the angle can be very tiny;essentially the surfaces should be nonparallel) to have more of a linecontact versus a plane contact.

Once poppet 32 is open, the surface tension of the oil and air interfacebecomes critical. If the space available around fingers 33 as it sits inopening 31 is too small, the surface tension of the oil will be so greatthat the bubble in chimney 26 cannot pass through the interface. Thedimensions for the preferred embodiment were determined experimentallyto avoid these concerns. In the preferred embodiment, opening 31 has adiameter of 0.22 inches, while the diameter of fingers 33 is 0.18 inchesand the space between fingers 33 is 0.10 inches. A further advantage ofthe present design is that the entire poppet 31 can move to one sideonce open, which allows a bigger space. In addition, the gap betweenfingers 33 provides an additional space for air to escape from chimney26.

The foregoing description of the preferred embodiment should not be readas limiting the invention, as other changes and modifications may bemade within the spirit and scope of this invention. This inventionshould be read as limited by the following claims only.

What is claimed is:
 1. A filter for a hydrostatic transmission assemblyhaving a housing forming an oil sump, said filter comprising: a filterhousing defining an internal volume; a filter mounted on said filterhousing for filtering oil passing from said oil sump into said internalvolume; a chimney, extending upwards from said filter housing, having anopening at the top thereof through which air is allowed to escape fromsaid chimney into said oil sump.
 2. A filter as set forth in claim 1,further comprising a poppet mounted in said opening in said chimney,said poppet having a top surface shaped to prevent contaminants in saidoil sump from entering said opening.
 3. A filter as set forth in claim2, wherein said chimney further comprises a lip formed around theperiphery of said chimney at the top thereof.
 4. A filter as set forthin claim 1, wherein the chimney is integrally formed as part of saidfilter housing.
 5. A filter as set forth in claim 3, wherein said filterhousing comprises: a top section having a generally flat upper surface,wherein said chimney extends upwards from and generally perpendicular tosaid top section, and said top section has a slanted surface extendingupwards from said generally flat upper surface to connect said flatupper surface and said chimney; and a bottom section on which saidfilter is mounted connected to said top section to form said interiorvolume.
 6. An axle driving apparatus comprising: a housing forming anoil sump; a filter housing secured in said housing defining an internalvolume; a filter mounted to said filter housing to filter oil passingfrom said sump to said internal volume; a center section secured in saidhousing having check openings to permit oil to pass from said internalvolume into hydraulic porting formed in said center section; a hydraulicpump and hydraulic motor rotatably mounted on said center section andconnected through said hydraulic porting; and a chimney formed adjacentto and extending upwardly from said internal volume of said filterhousing having an opening to permit air to move from said internalvolume to said sump.
 7. An axle driving apparatus as set forth in claim6, wherein said check openings are formed on the bottom of said centersection and said filter comprises a generally flat membrane secured tothe bottom of said filter housing.
 8. A filter as set forth in claim 7,wherein the chimney is integrally formed as part of said filter housingand is generally tubular in shape.
 9. A filter as set forth in claim 6,wherein the chimney is formed generally perpendicular to the top of saidfilter housing, and said filter housing further comprises a slantedsurface between the top of said filter housing and said chimney, whereinsaid slanted surface is not perpendicular to either said top surface orsaid chimney.
 10. An axle driving apparatus as set forth in claim 6,further comprising a poppet mounted in said opening in said chimney,said poppet having a top surface shaped to prevent contaminants in saidoil sump from entering said opening.
 11. A filter for a hydrostatictransmission assembly mounted in a housing forming an oil sump,comprising: a filter housing forming an internal volume; a filterassociated with said filter housing to filter oil passing from said sumpto said internal volume; and an opening at the top of said filterhousing to permit air to escape from said internal volume to said sump.12. A filter as set forth in claim 11, further comprising a poppetmounted in said opening, said poppet having a top surface shaped toprevent contaminants in said oil sump from entering said opening.
 13. Afilter as set forth in claim 12, further comprising a lip formed aroundthe periphery of said opening.
 14. A filter as set forth in claim 11,further comprising a chimney integrally formed with said filter housing,wherein said opening is located at the top of said chimney.
 15. A filteras set forth in claim 14, wherein said chimney is generally tubular inshape.
 16. A filter as set forth in claim 11, wherein said filter issecured directly to said filter housing.
 17. An axle driving apparatuscomprising: a housing forming an oil sump; a hydrostatic transmissionmounted in said housing and comprising a hydraulic pump and hydraulicmotor and connected through hydraulic porting, said hydraulic portinghaving at least one check opening; a structure secured in said housingand defining an internal volume having a first opening open to saidcheck openings and a second opening open to said sump; a filter locatedin said second opening to filter fluid passing from said oil sump intosaid internal volume; a chimney formed adjacent to and extendingupwardly from said internal volume of said structure and having anopening formed at the top of said chimney to permit air to move fromsaid internal volume to said sump.
 18. An axle driving apparatus as setforth in claim 17, wherein said chimney is integrally formed with saidstructure.
 19. A filter as set forth in claim 18, wherein said chimneyis generally tubular in shape.
 20. An axle driving apparatus as setforth in claim 17, wherein said filter is secured directly to saidstructure.
 21. An axle driving apparatus as set forth in claim 17,further comprising a poppet mounted in said opening in said chimney,said poppet having a top surface shaped to prevent contaminants in saidoil sump from entering said opening.
 22. An axle driving apparatus asset forth in claim 17, further comprising a lip formed at the top ofsaid chimney around said opening.